thermoplastic elastomers containing carbon nanotubes that is an autonomous, intrinsic, and reversible self healing polymer that requires no intervention to induce self healing, that requires no sequestered healing agents are added to the matrix, and is capable of multiple healing events.
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1. A self-healing polyurethane rubber, comprising:
a thermoplastic polyurethane synthesized from 3-methyl-1,5-pentanediol, poly(hexylmethylene carbonate)diol, a chain extender, and an isocyanante in the presence of a catalyst, and further comprising carbon nanotubes dispersed within the polyurethane at a concentration of about 0.25 percent to about 1 percent by weight, the polyurethane having stress at break of about 5 Mpa or greater and elongation at break of about 700 percent or greater after the polyurethane is separated and allowed to self-heal for about 24 hours.
4. The self-healing polyurethane rubber of
5. The self-healing polyurethane rubber of
6. The self-healing polyurethane rubber of
7. The self-healing polyurethane of
8. The self-healing polyurethane rubber of
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This application claims priority to U.S. Provisional Patent Application No. 61/305,370, entitled “ULTRA-SOFT POLYURETHANE CONTAINING CARBON NANOTUBES DEMONSTRATING SELF-HEALING PROPERTIES,” filed on Feb. 17, 2010, the contents of which are hereby incorporated by reference.
1. Field of the Invention
This invention relates to thermoplastic elastomers containing carbon nanotubes. More specifically, it relates to self-healing polycarbonate-containing polyurethane (PCU) nanotube composites.
2. Description of the Related Art
Elastomers, polymers commonly known as rubbers, are widely utilized because of their ability to recover shape and dimension after being subjected to deformation forces (stress). The cycle of stress and recovery can normally be repeated; however, all rubbers suffer from mechanical degradation over time and usually results in failure of the structural integrity of the material. Similarly, shape memory polymers are now used in a wide variety of applications, including wrinkle free clothing, robotics, and medical field. Shape memory polymers usually exist in one of two states: the native state; and a different shape that is induced by some sort of additional stimulus, e.g., heat, light, electromagnetic radiation etc.
What is needed is an autonomous, intrinsic, and reversible self healing polymer that requires no intervention to induce self healing, that requires no sequestered healing agents are added to the matrix, and is capable of multiple healing events.
The claimed invention is soft polyurethane incorporated with carbon nanotubes that has exceptional mechanical properties and shape memory. Once structural failure occurs (break or rupture) it is repaired by simply bringing surfaces of freshley broken segments in contact with one another. This material can be used anywhere soft polyurethanes are appropriate.
The claimed invention is a self-healing polycarbonate-containing polyurethane (PCU) nanotube composite. The composite is autonomous, meaning no intervention is needed to induce healing (such as heat, light, continuous pressure or electrical stimulus). Healing, however, is accelerated by heat. The composite is also intrinsic, meaning no sequestered healing agents are added to the matrix. Moreover, the healing is reversible, meaning the composites and neat polymer are capable of multiple healing events.
In an embodiment, the claimed invention includes XP-28203 having a plurality of carbon nanotubes disposed in its matrix. XP-28203 is a limited commercially available thermoplastic polyurethane synthesized from methylene bis(4-cyclohexylisocyanate), 1,4 butanediol, and a modified polycarbonate copolymer PES EX-619 (produced by Hodogaya Chemical Co., Ltd) and is manufactured by TSE Industries, in Clearwater Fla. This material was developed for its ultra-soft characteristics while retaining high mechanical properties. It was found that XP-28203 containing one percent by weight carbon nanotubes shows an increase in break strength while retaining its ultra-soft configuration (Shore A<70) and demonstrates self-healing characteristic. Upon breaking, tearing, or cutting formed samples of the polyurethane containing carbon nanotubes, the fragmented surfaces can be rejoined and form a cohesive complex that retains some of the initial mechanical properties.
For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which:
The claims invention is a thermoplastic elastomers that exhibit intrinsic self-healing that is enhanced by the incorporation of carbon nanotubes. The self-healing properties are enhanced by composites of the polycarbonate-containing polyurethane and single-wall (SW) or multi-wall (MW) carbon nanotubes (CNTs). The nanotubes are not used to encapsulate healing agents.
The polycarbonate polyol is unique in that it is a liquid at room temperature and the apparent lack of crystallinity leads to the creation of unusually soft polyurethanes (Shore A 60-70 hardness) while maintaining excellent physical and mechanical properties. The modified polyol of 2,000 a.m.u. contains 3-methyl-1,5-pentanediol. Appending a methyl group to the third position on pentanediol drastically alters the polyol. This modification impedes crystallization in the polyol and in the PCUs and significantly alters mechanical properties, shown in
Fully cured, intact surfaces do not adhere, however, once the surfaces are ruptured, the ruptured surfaces show a strong affinity for one another and when brought into contact self-mend.
A scale up batch of 23% hard segment PCU was used to produce single wall carbon nanotube (SWCNT) and multi-wall carbon nanotubes (MWCNT) composites containing 0.25 to 1.00% nanotubes by weight. Nanotubes were sonicated with polymer and tetrahydrofuran and the solvent was evaporated in a vacuum oven. Samples were molded to the appropriate geometry for testing. Differential scanning calorimetry was conducted on neat and filled samples. SWCNTs and MWCNTs had little or no effect glass transition temperatures (Table 1).
TABLE 1
PCPU Single & Multi-wall Cnt Composites
Activation
G′ (@15 Hz)
Energy
Sample
Tg ° C.
Tm ° C.
J/g
(−20° C.)(Mpa)
(kcal/mol)
Neat PU
−26
89
2.9
121.7
128
.25% SWCNT
−27
78
4.0
.50% SWCNT
−27
77
4.2
.75% SWCNT
−27
76
3.6
1.0% SWCNT
−27
76
3.6
231.8
136
.25% MWCNT
−27
62
3.8
.50% MWCNT
−27
63
3.7
.75% MWCNT
−27
63
4.4
1.0% MWCNT
−27
61
4.3
78.3
209
This indicates that there is not a strong association between the nanotubes and polymer, or, possibly that the polymer segments are so flexible that the nanotubes move in association with the segments (a 14° C. increase in the glass transition temperature was observed in poly methyl methacrylate (PMMA) upon the addition of 0.26% by weight of SWCNTs). Small, broad endotherms were noted in the melt region of the PUs. Melt temperatures decreased slightly with the addition of nanotubes and to a greater extent with SWCNTs, as shown in
Tensile tests were conducted at a crosshead speed of 4 inches per minute on the neat and composite samples. Five Samples were pulled for each formulation (Table 2).
TABLE 2
Tensile Test Results
Average stress
Average stress
Average stress
Average strain
Sample:
@100% strain (MpA)
@300% strain (MpA)
@break (Mpa)
@break ( % ini.)
Neat PCPU
1.34 +/− .09
1.90 +/− .15
3.52 +/− 0.45
583.0 +/− 25.70
0.25% SWCNT
3.02 +/− .15
6.57 +/− .35
9.00 +/− 0.30
445.4 +/− 12.76
0.50% SWCNT
3.00 +/− .31
5.97 +/− .46
8.10 +/− 0.52
441.4 +/− 28.20
0.75% SWCNT
2.36 +/− .06
5.30 +/− .15
6.90 +/− 0.87
434.0 +/− 60.26
1.0% SWCNT
2.64 +/− .11
5.80 +/− .27
7.70 +/− 0.28
442.4 +/− 20.90
0.25% MWCNT
2.50 +/− .14
4.59 +/− .16
11.30 +/− 1.90
459.0 +/− 28.20
0.50% MWCNT
3.24 +/− .15
7.22 +/− .32
9.80 +/− 0.49
444.2 +/− 12.40
0.75% MWCNT
2.96 +/− .09
6.33 +/− .14
8.20 +/− 0.35
439.2 +/− 10.90
1.0% MWCNT
3.06 +/− .17
6.53 +/− .27
8.50 +/− 0.37
443.8 +/− 22.70
An additional test was conducted to compare healing after samples were subjected
TABLE 3
Healed Tensile Test Results
Average stress
Average strain
Sample:
@break (Mpa)
@break (% ini.)
Neat PCPU Heal@RT
3.4 +/− 4.74
321.6 +/− 354.00
.25% SW Heal@RT
3.9 +/− 1.60
476.6 +/− 237.90
.50% SW Heal@RT
3.5 +/− 0.55
326.4 +/− 159.20
.75% SW Heal@RT
4.0 +/− 1.03
326.4 +/− 99.00
1% SW Heal@RT
4.1 +/− 1.30
324.2 +/− 159.20
.25% MW Heal@RT
3.5 +/− 1.10
620.8 +/− 262.70
.50% MW Heal@RT
4.6 +/− 1.23
435.6 +/− 142.50
.75% MW Heal@RT
4.0 +/− 1.50
346.4 +/− 217.60
1% MW Heal@RT
4.7 +/− 0.63
397.6 +/− 95.01
to tensile testing, pressed back together and left to mend at 25° C. for 24 hours. The results are reported in Table 3.
Only the ultimate stress at break is given for comparison for these samples. Key results are that the neat poly(carbonate urethane) self-heals. Further, strength is increased by both single and multi-wall CNTs. The stress values increased with nanotube level up to about 50% and then decreased.
Time effects were also explored by subjecting 1% single and multi-wall CNT PCU composites to rupture by cutting into two halves and then delaying the time the segments were brought into contact immediately (I), after 1 hour (1), 8 hours (8) and 24 hours (24) (Table 4).
TABLE 4
Separation Test Results
Sample:
Average stress
Average strain
all samples healed 24 h @ 25° C.
@break (Mpa)
@break(% ini.)
Immediate PCPU neat
4.22
735
Immediate SWCNT
5.43
752
Immediate MWCNT
5.23
758
1 hour wait PCPU neat
2.20
353
1 hour wait SWCNT
5.05
749
1 hour wait MWCNT
4.96
857
8 hour wait PCPU neat
1.13
58
8 hour wait SWCNT
2.09
230
8 hour wait MWCNT
1.73
194
24 hour wait PCPU neat
1.37
73
24 hour wait SWCNT
1.37
151
24 hour wait MWCNT
1.36
128
Single-wall CNT composites retained the ability to reheal after time delay better than the multi-wall CNT composites. It is interesting to note that even the 24 hour samples retained enough affinity to remain together after light pressure was applied.
The synthesis of a series of polymers with varying 3-methyl-1,5-pentanediol contents, varying hard segment contents, and varying nanotube contents yields leads to strong, reversible self-healing systems.
The claimed invention uses carbon nanotubes for reinforcement and to enhance self-healing. It does not use the nanotubes to release any type of healing agent into the matrix.
It is one object of the claimed invention to synthesize a systematic series of linear thermoplastic polyurethanes using the starting materials in table 5.
TABLE 5
Raw materials
Ratio %
Polyols
1,6 Hexane diol/3-Me-1,5 Pentane diol
70/30
Poly (Hexylmethylene carbonate) diol
Chain extender
1,4 butane diol
2-methyl-1,3-propane diol
1,5 pentane diol
2-methyl-1,4-butane diol
Isocyanate
Monomeric Diphenylmethane 4,4′-Diisocyanate
bis(4-isocyanotocyclohexyl) methane
Catalyst
Tin 2-ethylhexanoate
A novel polycarbonate polyol containing 2-methyl-1,5-pentane diol was found to yield ultra-soft, non-blocking polyurethanes. The low hardness comes about by minimizing crystallinity by the appended methyl group in the diol. However, it is important to demonstrate the effect of hard segment content, polyol chemistry and chain extender on self-healing properties. The hard segment chemistry testing chain extenders in additional to 1,4-butane diol was also altered.
It is a further object to process MWCNT and SWCNT composites and characterize the neat polymers and nanocomposites.
Synthesize of a systematic series of linear thermoplastic polyurethanes. Table 5 lists the polyols, chain extenders, isocyanate and catalyst used in this experiment. The original polymer contained 23% hard segment (molar amounts of methylene diphenyl isocyanate and 1,4 butane diol). The polyether polyol (6,000 molar mass) contained 1,6-hexane diol and 3-methyl-1,5-pentanediol in a 70/30 mole ratio. The 3-methyl-1,5-pentanediol inhibited crystallization in the polycarbonate diol. The resulting polyurethane (60,000 g mol−1) was never optimized for self-healing applications. The following will be varied:
Polycarbonate copolymer is charged into a reactor equipped with constant nitrogen blanketing and a heating mantle with controlled temperature and mixing, the temperature is maintained between 60 and 80° C. The low molecular weight chain extender is then added and mixed. The catalyst, tin 2-ethyl hexanoate is added, stirred and a slight stoichiometric excess of diisocyanate is added. The resulting polyurethane is placed in an oven at 107° C. for two hours for curing and then the temperature is dropped to 93° C. for post curing. The post curing is continued until the absence of isocyanate groups absorption at 2264 cm−1 is confirmed by FTIR spectrometry. All samples were aged seven days before continuing.
Two Shot Method
Step 1:
The diisocyanate is charged into a reactor equipped with constant nitrogen blanketing and a heating mantle with controlled temperature and mixing. The polycarbonate copolymer is then added to the isocyanate targeting the desired % NCO. A small amount of catalyst is added. The mixture is heated to 100° C. and allowed to react for several hours. The result is an isocyanate terminated prepolymer.
Step 2
The polycarbonate copolymer is charged into a 2 L reactor equipped with constant nitrogen blanketing and a heating mantle with controlled temperature and mixing, and the temperature is maintained between 60-80° C. The low molecular weight chain extender is then added and mixed. A small amount of tin 2-ethyl hexanoate is then added. The prepolymer is then added to this mixture in a slight stoichiometric excess and mixed thoroughly. The resulting thermoplastic urethane is then placed in an oven at 107° C. for two hours for curing and then the temperature is dropped to 93° C. while post curing. The post curing is continued until the absence of isocyanate groups absorption at 2264 cm−1 is confirmed by FTIR spectrometry. All samples were aged seven days before continuing.
Note: PUs prepared via these methods appears to dissolve and produce clear solutions. However, small branched components have been isolated via a 2 micron filter. It is known that, for example, allophanates form during these reactions. More so, at higher temperatures. The will be monitored. Catalyst and temperature will be adjusted to minimize this. It is likely than branching effects the self-healing process.
MWCNT and SWCNT composites are processed. AP grade SWCNT (1.20-1.5 nm D) and MWCNT (30-50 nm O.D. and 5-15 I.D.) will be used as received from Sigma. PU, 5% by weight) will be dissolved in tetrahydrofuran (THF) under stirring for 6 hours. Nanotubes will be added and the mixture (0.10-1.0% by weight) will be sonicated for 2 hours with a Branson Sonifier 250. The mixture will be poured in a shallow pan on a shaker (200 RPM) and placed in a hood until the THF evaporates. Samples will be removed from the pan and vacuum dried for 12 hours at 50°. Note: IF scanning electron microscopy reveals poor dispersion a rapid solvent casting method will be used to enhance dispersion.
Carbonate polyols made using any carbonates listed in Table 6 with any of the diols listed under chain extenders.
##STR00001##
TABLE 6
R1
R2
R3
R4
R5
R6
Bp (° C.)
Yield (%)
CH3
CH3
H
H
CH3
H
197
39.8
CH(CH3)2
H
CH3
CH3
H
H
232
59.6
CH3
H
H
H
H
H
203
35.1
H
H
C2H5
C4H9
H
H
262
59.6
C3H7
H
C2H5
H
H
H
244
76.2
Chain Extenders:
1.
Ethane-1,2-diol (ethylene glycol, glycol), C2H4O2, HO—CH2—CH2—OH, ##STR00002##
2.
Propane-1,2-diol, ##STR00003##
3.
Propane-1,3-diol, HO—CH2—CH2—CH2—OH, ##STR00004##
4.
Propane-1,2,3-triol (glycerol), ##STR00005##
5.
Butane-1,2-diol, ##STR00006##
6.
Butane-1,3-diol, ##STR00007##
7.
Butane-1,4-diol, HO—CH2—CH2—CH2—CH2—OH, ##STR00008##
8.
Butane-2,3-diol, ##STR00009##
9.
Butane-1,2,3-triol, ##STR00010##
10.
Butane-1,2,4-triol, ##STR00011##
11.
Pentane-1,2-diol, ##STR00012##
12.
Pentane-1,3-diol, ##STR00013##
13.
Pentane-1,4-diol, ##STR00014##
14.
Pentane-1,5-diol, HO—CH2—CH2—CH2—CH2—CH2—OH, ##STR00015##
15.
Pentane-2,3-diol, ##STR00016##
16.
Pentane-2,4-diol, ##STR00017##
17.
Hexane-2,3-diol, ##STR00018##
18.
Hexane-2,5-diol, ##STR00019##
19.
Cyclopentanol (cyclopentyl alcohol), ##STR00020##
20.
Cyclopentane-1,2-diol, ##STR00021##
21.
Cyclopentane-1,3-diol, ##STR00022##
22.
diol, ##STR00023##
23.
Cyclohexane-1,3-diol, ##STR00024##
24.
Cyclohexane-1,4-diol, ##STR00025##
Diisocyanate Structure:
Name
Structure
1,6- hexamethylene diisocyanate (HDI)
##STR00026##
isophorone diisocyanate (IPDI)
##STR00027##
4,4′-dicyclo- hexylmethane diisocyanate (H12MDI) *
##STR00028##
1,4-cyclohexane diisocyanate (CHDI) *
##STR00029##
bis(isocyanato- methyl)cyclo- hexane (H4XDI,DDI) *
##STR00030##
Carbonate Polyols
Polycabonante polyols made using any combination of the glycols listed in the chain extender section.
Nanofillers:
It will thus be seen that the objects set forth above, and those made apparent from the foregoing disclosure, are efficiently attained. Since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing disclosure or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein disclosed, and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween.
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